Acoustic wave sensors are utilized in a variety of sensing applications, such as, for example, temperature and/or pressure sensing devices and systems. Acoustic wave devices have been in commercial use for over sixty years. Although the telecommunications industry is the largest user of acoustic wave devices, they are also used for sensor applications, such as in chemical vapor detection. Acoustic wave sensors are so named because they use a mechanical, or acoustic, wave as the sensing mechanism. As the acoustic wave propagates through or on the surface of the material, any changes to the propagation path affect the characteristics of the wave.
Changes in acoustic wave characteristics can be monitored by measuring the frequency or phase characteristics of the sensor and can then be correlated to the corresponding physical quantity or chemical quantity that is being measured. Virtually all acoustic wave devices and sensors utilize a piezoelectric crystal (e.g., quartz), a semi-crystalline piezoelectric materials (e.g., PVDF), or an amorphous piezoelectric materials to generate the acoustic wave. Three mechanisms can contribute to acoustic wave sensor response, i.e., mass-loading, visco-elastic and acousto-electric effect. The mass-loading of chemicals alters the frequency, amplitude, and phase and Q value of such sensors. Most acoustic wave chemical detection sensors, for example, rely on the mass sensitivity of the sensor in conjunction with a chemically selective coating that absorbs the vapors of interest resulting in an increased mass loading of the acoustic wave sensor.
Examples of acoustic wave sensors include acoustic wave detection devices, which are utilized to detect the presence of substances, such as chemicals, or environmental conditions such as temperature and pressure. An acoustical or acoustic wave (e.g., SAW/BAW) device acting as a sensor can provide a highly sensitive detection mechanism due to the high sensitivity to surface loading and the low noise, which results from their intrinsic high Q factor. Surface acoustic wave devices are typically fabricated using photolithographic techniques with comb-like interdigital transducers placed on a piezoelectric material. Surface acoustic wave devices may have either a delay line or a resonator configuration. Bulk acoustic wave devices are typically fabricated using a vacuum plater, such as those made by CHA, Transat or Saunder. The choice of the electrode materials and the thickness of the electrode are controlled by filament temperature and total heating time. The size and shape of electrodes are defined by proper use of masks.
Surface acoustic wave resonator (SAW-R), surface acoustic wave delay line (SAW-DL), surface transverse wave (STW), bulk acoustic wave (BAW), flexural plate wave (FPW), and acoustic plate mode (APM) all can be utilized in various sensing measurement applications. One of the primary differences between an acoustic wave sensor and a conventional sensor is that an acoustic wave sensor can store energy mechanically. Once such a sensor is supplied with a certain amount of energy (e.g., through RF), the sensor can operate for a time without any active part (e.g., without a power supply or oscillator). This feature makes it possible to implement an acoustic wave sensor in an RF powered passive and wireless sensing application.
One field where acoustic wave devices may offer a promising future is the area lubricity measurements. Lubricity is an important quality factor for diesel fuels due to the reduction lubricity associated with the extreme hydrogenation needed to obtain the low sulfur levels required in modern diesel fuels. In general, diesel engines rely on fuel as a lubricant for their internal moving components. The lubricity of the fuel affects the wear between two metal parts that are in contact. Wear due to the friction between these parts will cause failure of the components if there is insufficient lubricity. The use of a high lubricity fuel may reduce the wear and increase component life.
Sulfur is found naturally in crude oil and carries through into diesel and gasoline fuels unless specifically removed through distillation. As a result, diesel and gasoline fuels used in engines may contain sulfur in concentrations up to 3000 parts per million (ppm). At such high concentrations, sulfur provides high lubricity in fuel pumps and injector systems that deliver the fuel to the combustion chamber in an engine. Fuel sulfur, however, also causes polluting emissions, particularly SO2 and soot particles, and poisons the advanced emission-control and after treatment devices that are being developed to enable diesel engines to meet progressively more stringent emissions standards. Sulfur dioxide emissions are associated with environmental problems such as acid rain. However, when the current sulfur level is reduced in fuels, high friction and wear occur on sliding surfaces of fuel delivery systems and cause catastrophic failure.
Fuels with lower sulfur content have lower lubricity compared to those with higher sulfur content. Thus, low-sulfur diesel fuels do not provide sufficient lubricity for use in diesel engines, and the use of low-sulfur diesel fuels results in high friction and catastrophic wear of fuel pumps and injectors. When lubricity is compromised, wear increases in fuel injection systems, most of which were originally designed with the natural lubricating properties of traditional diesel fuel in mind.
The lower lubricity of low-sulfur fuels poses significant problems for producers as well as for end-users of diesel fuels. Reduction in lubricity also contributes to a loss in usable power due to the increased friction the engine has to overcome. Because fuels with lower sulfur contents exhibit increased friction characteristics compared to fuels with higher sulfur contents, a perfectly tuned engine experiences a noticeable drop in efficiency when the fuel is changed from a high-sulfur fuel to a low-sulfur fuel.
Traditional lubricity is tested by utilizing the Scuffing Load Ball on Cylinder (SLBOCLE) and the High Frequency Reciprocating Rig Test (HFRR) methods. In general, the SLBOCLE test measures the maximum load a ball on rotating cylinder can sustain without experiencing scuffing wear. In the HFRR testing technique, a hardened steel ball can oscillate across a hardened steel plate under a fixed load for a particular amount of time (e.g., 75 minutes). The point of contact between the ball and plate is immersed in a sample. The size of the resulting wear scar on the steel ball is a measure of the sample's lubricity. Such testing methodologies are large, expensive and time-consuming.